Molar Mass
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol), and numerically equal to the substance's relative atomic or molecular mass in unified atomic mass units. It is calculated by summing the atomic masses of all atoms in one formula unit of the substance, using values from the periodic table. Molar mass is the essential conversion factor between grams (measurable in the lab) and moles (used in chemical calculations).
Key Formula
M = m / n
LaTeX: M = \dfrac{m}{n}
| Symbol | Meaning | Unit |
|---|---|---|
| M | Molar mass | g/mol |
| m | Mass of the substance | g |
| n | Amount of substance | mol |
Worked Example
Problem
Calculate the molar mass of glucose (C₆H₁₂O₆) and find the number of moles in 90.0 g.
Solution
Step 1: Find molar mass of C₆H₁₂O₆: C: 6 × 12.011 = 72.066 g/mol H: 12 × 1.008 = 12.096 g/mol O: 6 × 15.999 = 95.994 g/mol Total M = 72.066 + 12.096 + 95.994 = 180.156 g/mol. Step 2: Calculate moles: n = m / M = 90.0 g / 180.156 g/mol = 0.4996 mol.
Answer
Molar mass of glucose = 180.16 g/mol; n ≈ 0.500 mol in 90.0 g
Molar Masses of Common Substances
| Substance | Formula | Molar Mass (g/mol) | Common Use |
|---|---|---|---|
| Water | H₂O | 18.015 | Universal solvent |
| Sodium chloride | NaCl | 58.44 | Table salt |
| Carbon dioxide | CO₂ | 44.010 | Greenhouse gas |
| Ethanol | C₂H₅OH | 46.068 | Alcohol in beverages |
| Calcium carbonate | CaCO₃ | 100.09 | Limestone, antacid |
| Glucose | C₆H₁₂O₆ | 180.16 | Primary biological fuel |
Interactive Tools
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Related Terms
Mole (chemistry)
The mole is the SI base unit for the amount of substance, defined as exactly 6.02214076 × 10²³ elementary entities (atoms, molecules, ions, or other particles). It provides chemists with a practical bridge between the atomic scale and macroscopic, measurable quantities in the laboratory. One mole of any substance contains the same number of particles, making it the universal counting unit of chemistry.
Stoichiometry
Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It uses balanced chemical equations to calculate the masses, volumes, or moles of substances involved in a reaction. Stoichiometry is fundamental to industrial chemistry, pharmaceutical manufacturing, and any process requiring precise control of chemical quantities.
Molarity
Molarity (M) is the most common measure of solution concentration, defined as the number of moles of solute dissolved per litre of solution. It is temperature-dependent because liquid volumes change with temperature. Molarity is widely used in titrations, reaction stoichiometry involving solutions, and the preparation of standard laboratory solutions.
From Latin "moles" (mass, pile) and "massa" (mass). The concept developed throughout the 19th century as atomic theory matured; the standardised "molar mass" terminology was formalised with the adoption of the SI in 1971.